The invention relates to an ECG electrode structure suitable to be used in water during swimming, for example, and to a method for measuring an ECG signal from a person who is in water.
Heart rate measurement is based on monitoring the operation of the heart. When heart contracts, it causes a series of electric pulses in the body that can be measured. The measurement and analysis of a signal thus caused is known as electrocardiography (ECG). The signal itself is called an ECG signal. Further details about ECG can be obtained from Guyton, Arthur C.: Human Physiology and Mechanisms of Disease, Third edition, W. B. Saunders Company 1982, ISBN 4-7557-0072-8, Chapter 13: The Electrocardiogram, which is included herein by reference.
U.S. Pat. No. 4,625,733, Sxc3xa4ynxc3xa4jxc3xa4kangas, teaches a wireless and continuous heart rate measurement concept consisting of a measuring belt to be positioned on the chest by means of an elastic band and a heart rate receiver worn on the wrist like a watch.
The measuring belt consists of a bending piece attached to the chest with an elastic belt, the piece comprising two measurement electrodes that set against the skin and an ECG detection block connected to the measurement electrodes for generating heart rate information based on the ECG signal measurements made by the measurement electrodes. The transmitter on the measuring belt uses inductive telemetry to transmit the heart rate information to a heart rate receiver attached to the user""s wrist.
The measurement electrodes detect a projection of the heart""s electric field on the skin. Although both the electrodes are measurement electrodes by structure, the electrode which is on the left-hand side of the measuring belt and partly sets on the heart is usually the actual measurement electrode and the right-hand side electrode is a reference or ground electrode. The measurement is carried out for measuring a difference in potential, i.e. voltage, between the measurement electrodes in question. From the point of view of the measurement, the only important aspect is the potential difference between the first and the second electrode.
Further details about the electronics needed in the measurement can be obtained from Bronzino, Joseph D.: The Biomedical Engineering Handbook, CRC Press 1995, Chapter 72: Biopotential Amplifiers which is included herein by reference.
In order for the described measurement electrode structure to be applicable also during swimming, the measurement electrodes must be insulated from water. The insulation can be carried out by arranging a protrusion around the measurement electrodes to prevent water from entering between the measurement electrode and the skin when the belt is placed against the skin. However, this is not a particularly good solution because when the swimmer jumps into water or during swimming, the movement of the muscles causes the belt to move whereby water gets between the measurement electrode and the skin. In addition, movement of the chest caused by breathing may cause the belt to move.
In a way, water can be considered to serve as a third electrode because, except for the body areas that are under the measurement electrodes, the body is in contact with the water which is usually conductive due to the impurities it contains. If both the measurement electrodes in the measurement belt get into contact with the water, the measurement electrodes are, in an electric sense, shortcut with respect to each other and the potential difference between them can no longer be measured. If only the first one of the measurement electrodes in the measuring belt gets into contact with water, then the potential difference between the electrode consisting of the first measurement electrode and the water and the second measurement electrode is measured. When the first measurement electrode changes to an electrode consisting of the first measurement electrode and the water, the projection of the heart""s electric field changes, thereby interfering with the measurement accuracy.
The electrodes can also be attached using adhesive tape or glue, but they are not very pleasant to use. In addition, glue may soil the water.
U.S. Pat. Nos. 4,637,399 and 4,791,933 describe electrode structures to be attached to the surface of the skin in a waterproof manner. The fastening is based on the use of a suction disc. The solution is not very pleasant to use, because the suction disc causes a negative pressure which draws the skin to it, thereby possibly causing redness of the skin.
It is an object of the invention to provide an improved ECG electrode structure and an improved method for measuring an ECG signal from a person who is in water. One aspect of the invention is an ECG electrode structure according to claim 1. Another aspect of the invention is a method according to claim 16 for measuring an ECG signal from a person who is in water. Other preferred embodiments of the invention are disclosed in the dependent claims.
An underlying idea of the invention is that, unlike in the prior art, the second measurement electrode is arranged to be in contact only with the water and not with the skin. Since this solution requires only one measurement electrode which is electrically insulated from water, the positioning of the electrode and its insulation are easier to carry out than in a solution where two measurement electrodes are to be positioned on the skin. This improves the reliability of the measurement.